KDTree VS LiteNetLib

This implementation has both knearest and radius query functions.

I personally use this LiteNetLib

Been using LiteNetLib for our current project and it is pretty nice

Currently I'm using LiteNetLib (https://github.com/RevenantX/LiteNetLib) and it's working pretty well. There is a little input lag but it's more than acceptable.

Check LiteNetLib. Judging by your requirements simply passing coordinates back and forth between two clients should work. Designate one of the games as host and the other as client, but just pass coordinates would work over LAN. Optimise from there if gameplay isn’t thet smoot by lerping between past and newly received coordinates.

For networking, file IO, and streams in general, there's Korio and for Java; for just networking, there's LiteNetLib for C#; for what looks like data streams in general, there's Okio also for Java; and Tokio for multi-threaded IO in Rust.

I've been a bit "Unity offline", and even more so when it comes to networking solutions, because of my real job :), but LiteNetLib is quite good. There's also a new HLAPI solution called FishNet which seems very interesting.

This might be handy for you :) https://github.com/RevenantX/LiteNetLib

they might be using LiteNetLib as their low level transport.

You definitely want to a low level API (LLAPI) instead, like f.ex. LiteNetLib, ENet, lidgren or similar. I haven't touched anyone of these recently, except the first one a year-ish ago, so I won't say anything about where they stand today.